Composite polyamide membrane made via interfacial polymerization using a blend of non-polar solvents

ABSTRACT

A method for making a composite polyamide membrane comprising a porous support and a thin film polyamide layer, wherein the method includes: i) applying a polar solution comprising a polyfunctional amine monomer, and a non-polar solution comprising a polyfunctional acyl halide monomer to a surface of a porous support and interfacially polymerizing the monomers to form a thin film polyamide layer, wherein the non-polar solution further comprises at least 50 vol % of a C 5  to C 20  aliphatic hydrocarbon and from 2 to 25 vol % of benzene or benzene substituted with one or more C 1  to C 6  alkyl groups; and ii) applying an aqueous solution of nitrous acid to the thin film polyamide layer.

FIELD

The present invention is generally directed toward composite polyamidemembranes along with methods for making and using the same.

INTRODUCTION

Composite polyamide membranes are used in a variety of fluidseparations. One common class of membranes includes a porous supportcoated with a “thin film” polyamide layer. The thin film layer may beformed by an interfacial polycondensation reaction betweenpolyfunctional amine (e.g. m-phenylenediamine) and polyfunctional acylhalide (e.g. trimesoyl chloride) monomers which are sequentially coatedupon the support from immiscible solutions, see for example U.S. Pat.No. 4,277,344 to Cadotte. Various constituents may be added to one orboth of the coating solutions to improve membrane performance. Forexample, U.S. Pat. No. 4,259,183 to Cadotte describes the use ofcombinations of bi- and tri-functional acyl halide monomers, e.g.isophthaloyl chloride or terephthaloyl chloride with trimesoyl chloride.U.S. Pat. No. 6,878,278 to Mickols describes the addition of a widerange of complexing agents to the acyl halide coating solution,including various tri-hydrocarbyl phosphate compounds. US 2011/0049055describes the addition of moieties derived from sulfonyl, sulfinyl,sulfenyl, sulfuryl, phosphoryl, phosphonyl, phosphinyl, thiophosphoryl,thiophosphonyl and carbonyl halides. U.S. Pat. No. 6,521,130 describesthe addition of a carboxylic acid (e.g. aliphatic and aromaticcarboxylic acids) or carboxylic acid ester to one or both monomercoating solutions prior to polymerization. Similarly, U.S. Pat. No.6,024,873, U.S. Pat. No. 5,989,426, U.S. Pat. No. 5,843,351 and U.S.Pat. No. 5,576,057 describe the addition of selected alcohols, ethers,ketones, esters, halogenated hydrocarbons, nitrogen-containing compoundsand sulfur-containing compounds having solubility parameters of 8 to 14(cal/cm³)^(1/2) to one of the coating solutions. US 2009/0107922describes the addition of various “chain capping reagents” to one orboth coating solutions, e.g. 1,3 propane sultone, benzoyl chloride,1,2-bis(bromoacetoxy) ethane, etc. U.S. Pat. No. 4,606,943 and U.S. Pat.No. 6,406,626 describe the formation of a thin film polyamide using apolyfunctional amine and polyfunctional acyl halide along with apolyfunctional acid anhydride halide (e.g. trimellitic anhydride acylchloride). WO2012/102942, WO2012/102943 and WO2012/102944 describe theaddition of various monomers including carboxylic acid andamine-reactive functional groups. US 2009/0272692, US 2012/0261344 andU.S. Pat. No. 8,177,978 describe the use of various polyfunctional acylhalides and their corresponding partially hydrolyzed counterparts. U.S.Pat. No. 4,812,270 and U.S. Pat. No. 4,888,116 to Cadotte (see also WO2012/090862, WO 2012/020680, WO 2011/105278, WO 2011/078131 and WO2011/078047) describe post-treating the membrane with phosphoric ornitrous acid. U.S. Pat. No. 5,582,725 describes a similar post treatmentwith an acyl halide such as benzoyl chloride. The search continues fornew combinations of monomers, additives and post-treatments that furtherimprove membrane performance.

SUMMARY

The invention includes a method for making a composite polyamidemembrane including a porous support and a thin film polyamide layer. Themethod includes the steps of:

-   -   i) applying a polar solution comprising a polyfunctional amine        monomer, and a non-polar solution comprising a polyfunctional        acyl halide monomer to a surface of a porous support and        interfacially polymerizing the monomers to form a thin film        polyamide layer,    -   wherein the non-polar solution further comprises at least 50 vol        % of a C₅ to C₂₀ aliphatic hydrocarbon and from 2 to 25 vol % of        benzene or benzene substituted with one or more C₁ to C₆ alkyl        groups; and    -   ii) applying an aqueous solution of nitrous acid to the thin        film polyamide layer.        Many additional embodiments are described including applications        for such membranes.

DETAILED DESCRIPTION

The invention is not particularly limited to a specific type,construction or shape of composite membrane or application. For example,the present invention is applicable to flat sheet, tubular and hollowfiber polyamide membranes useful in a variety of applications includingforward osmosis (FO), reverse osmosis (RO), nano filtration (NF), ultrafiltration (UF), micro filtration (MF) and pressure retarded fluidseparations. However, the invention is particularly useful for membranesdesigned for RO and NF separations. RO composite membranes arerelatively impermeable to virtually all dissolved salts and typicallyreject more than about 95% of salts having monovalent ions such assodium chloride. RO composite membranes also typically reject more thanabout 95% of inorganic molecules as well as organic molecules withmolecular weights greater than approximately 100 Daltons. NF compositemembranes are more permeable than RO composite membranes and typicallyreject less than about 95% of salts having monovalent ions whilerejecting more than about 50% (and often more than 90%) of salts havingdivalent ions—depending upon the species of divalent ion. NF compositemembranes also typically reject particles in the nanometer range as wellas organic molecules having molecular weights greater than approximately200 to 500 Daltons.

Examples of composite polyamide membranes include FilmTec CorporationFT-30™ type membranes, i.e. a flat sheet composite membrane comprising abottom layer (back side) of a nonwoven backing web (e.g. PET scrim), amiddle layer of a porous support having a typical thickness of about25-125 μm and top layer (front side) comprising a thin film polyamidelayer having a thickness typically less than about 1 micron, e.g. from0.01 micron to 1 micron but more commonly from about 0.01 to 0.1 μm. Theporous support is typically a polymeric material having pore sizes whichare of sufficient size to permit essentially unrestricted passage ofpermeate but not large enough so as to interfere with the bridging overof a thin film polyamide layer formed thereon. For example, the poresize of the support preferably ranges from about 0.001 to 0.5 μm.Non-limiting examples of porous supports include those made of:polysulfone, polyether sulfone, polyimide, polyamide, polyetherimide,polyacrylonitrile, poly(methyl methacrylate), polyethylene,polypropylene, and various halogenated polymers such as polyvinylidenefluoride. For RO and NF applications, the porous support providesstrength but offers little resistance to fluid flow due to itsrelatively high porosity.

Due to its relative thinness, the polyamide layer is often described interms of its coating coverage or loading upon the porous support, e.g.from about 2 to 5000 mg of polyamide per square meter surface area ofporous support and more preferably from about 50 to 500 mg/m². Thepolyamide layer is preferably prepared by an interfacialpolycondensation reaction between a polyfunctional amine monomer and apolyfunctional acyl halide monomer upon the surface of the poroussupport as described in U.S. Pat. No. 4,277,344 and U.S. Pat. No.6,878,278. More specifically, the polyamide membrane layer may beprepared by interfacially polymerizing a polyfunctional amine monomerwith a polyfunctional acyl halide monomer, (wherein each term isintended to refer both to the use of a single species or multiplespecies), on at least one surface of a porous support. As used herein,the term “polyamide” refers to a polymer in which amide linkages(—C(O)NH—) occur along the molecular chain. The polyfunctional amine andpolyfunctional acyl halide monomers are most commonly applied to theporous support by way of a coating step from solution, wherein thepolyfunctional amine monomer is typically coated from an aqueous-basedor polar solution and the polyfunctional acyl halide from anorganic-based or non-polar solution. Although the coating steps need notfollow a specific order, the polyfunctional amine monomer is preferablyfirst coated on the porous support followed by the polyfunctional acylhalide. Coating can be accomplished by spraying, film coating, rolling,or through the use of a dip tank among other coating techniques. Excesssolution may be removed from the support by air knife, dryers, ovens andthe like.

The polyfunctional amine monomer comprises at least two primary aminegroups and may be aromatic (e.g., m-phenylenediamine (mPD),p-phenylenediamine, 1,3,5-triaminobenzene, 1,3,4-triaminobenzene,3,5-diaminobenzoic acid, 2,4-diaminotoluene, 2,4-diaminoanisole, andxylylenediamine) or aliphatic (e.g., ethylenediamine, propylenediamine,cyclohexanne-1,3-diameine and tris (2-diaminoethyl) amine). Oneparticularly preferred polyfunctional amine is m-phenylene diamine(mPD). The polyfunctional amine monomer may be applied to the poroussupport as a polar solution. The polar solution may contain from about0.1 to about 10 wt % and more preferably from about 1 to about 6 wt %polyfunctional amine monomer. In one set of embodiments, the polarsolutions includes at least 2.5 wt % (e.g. 2.5 to 6 wt %) of thepolyfunctional amine monomer. Once coated on the porous support, excesssolution may be optionally removed.

The polyfunctional acyl halide monomer comprises at least two acylhalide groups and preferably no carboxylic acid functional groups andmay be coated from a non-polar solvent although the polyfunctional acylhalide may be alternatively delivered from a vapor phase (e.g., forpolyfunctional acyl halides having sufficient vapor pressure). Thepolyfunctional acyl halide is not particularly limited and aromatic oralicyclic polyfunctional acyl halides can be used along withcombinations thereof. Non-limiting examples of aromatic polyfunctionalacyl halides include: trimesoyl chloride, terephthalic acyl chloride,isophthalic acyl chloride, biphenyl dicarboxylic acyl chloride, andnaphthalene dicarboxylic acid dichloride. Non-limiting examples ofalicyclic polyfunctional acyl halides include: cyclopropane tricarboxylic acyl chloride, cyclobutane tetra carboxylic acyl chloride,cyclopentane tri carboxylic acyl chloride, cyclopentane tetra carboxylicacyl chloride, cyclohexane tri carboxylic acyl chloride, tetrahydrofurantetra carboxylic acyl chloride, cyclopentane dicarboxylic acyl chloride,cyclobutane dicarboxylic acyl chloride, cyclohexane dicarboxylic acylchloride, and tetrahydrofuran dicarboxylic acyl chloride. One preferredpolyfunctional acyl halide is trimesoyl chloride (TMC). Thepolyfunctional acyl halide may be dissolved in a non-polar solvent in arange from about 0.01 to 10 wt %, preferably 0.05 to 3% wt % and may bedelivered as part of a continuous coating operation. In one set ofembodiments wherein the polyfunctional amine monomer concentration isless than 3 wt %, the polyfunctional acyl halide is less than 0.3 wt %.

The non-polar solution comprises a blend of aliphatic and aromaticsolvents. More specifically, the non-polar solution comprises from 2 to25 vol % (and more preferably from 3 to 15 vol %) of benzene or benzenesubstituted with one or more C₁-C₆ alkyl groups as represented inFormula (I).

wherein R₁ through R₆ independently selected from hydrogen or an alkylgroup having from 1 to 6 carbon atoms. In a preferred set ofembodiments, the benzene ring is substituted with a plurality of methylgroups, e.g. 1,3,5-trimethylbenzene (“mesitylene”). The non-polarsolution further comprises at least 50 vol % (e.g. from 50 to 98 vol %and more preferably from 50 to 90 vol %) of a C₅ to C₂₀ aliphatichydrocarbon solvent. Representative examples include paraffins (e.g.hexane, cyclohexane, heptane, octane, dodecane) and isoparaffins (e.g.ISOPAR™ L).

In a preferred subset of embodiments, the non-polar solution furthercomprises an acid-containing monomer comprising a C₂-C₂₀ hydrocarbonmoiety substituted with at least one carboxylic acid functional group orsalt thereof and at least one amine-reactive functional group selectedfrom: acyl halide, sulfonyl halide and anhydride, wherein theacid-containing monomer is distinct from the polyfunctional acyl halidemonomer. In one set of embodiments, the acid-containing monomercomprises an arene moiety. Non-limiting examples include mono anddi-hydrolyzed counterparts of the aforementioned polyfunctional acylhalide monomers including two to three acyl halide groups and mono, diand tri-hydrolyzed counterparts of the polyfunctional halide monomersthat include at least four amine-reactive moieties. A preferred speciesincludes 3,5-bis(chlorocarbonyl)benzoic acid (i.e. mono-hydrolyzedtrimesoyl chloride or “mhTMC”). Additional examples of monomers aredescribed in WO 2012/102942 and WO 2012/102943 (see Formula III whereinthe amine-reactive groups (“Z”) are selected from acyl halide, sulfonylhalide and anhydride). Specific species including an arene moiety and asingle amine-reactive group include: 3-carboxylbenzoyl chloride,4-carboxylbenzoyl chloride, 4-carboxy phthalic anhydride and 5-carboxyphthalic anhydride, and salts thereof. Additional examples arerepresented by Formula (II):

wherein A is selected from: oxygen (e.g. —O—); amino (—N(R)—) wherein Ris selected from a hydrocarbon group having from 1 to 6 carbon atoms,e.g. aryl, cycloalkyl, alkyl—substituted or unsubstituted but preferablyalkyl having from 1 to 3 carbon atoms with or without substituents suchas halogen and carboxyl groups); amide (—C(O)N(R))— with either thecarbon or nitrogen connected to the aromatic ring and wherein R is aspreviously defined; carbonyl (—C(O)—); sulfonyl (—SO₂—); or is notpresent (e.g. as represented in Formula III); n is an integer from 1 to6, or the entire group is an aryl group; Z is an amine reactivefunctional group selected from: acyl halide, sulfonyl halide andanhydride (preferably acyl halide); Z′ is selected from the functionalgroups described by Z along with hydrogen and carboxylic acid. Z and Z′may be independently positioned meta or ortho to the A substituent onthe ring. In one set of embodiments, n is 1 or 2. In yet another set ofembodiments, both Z and Z′ are both the same (e.g. both acyl halidegroups). In another set of embodiments, A is selected from alkyl andalkoxy groups having from 1 to 3 carbon atoms. Non-limitingrepresentative species include: 2-(3,5-bis(chlorocarbonyl)phenoxy)aceticacid, 3-(3,5-bis(chlorocarbonyl)phenyl) propanoic acid,2-((1,3-dioxo-1,3-dihydroisobenzofuran-5-yl)oxy)acetic acid,3-(1,3-dioxo-1,3-dihydroisobenzofuran-5-yl)propanoic acid,2-(3-(chlorocarbonyl) phenoxy)acetic acid,3-(3-(chlorocarbonyl)phenyl)propanoic acid,3-((3,5bis(chlorocarbonyl)phenyl) sulfonyl) propanoic acid,3-((3-(chlorocarbonyl)phenyl)sulfonyl)propanoic acid,3-((1,3-dioxo-1,3-dihydroisobenzofuran-5-yl)sulfonyl)propanoic acid,3-((1,3-dioxo-1,3-dihydroisobenzofuran-5-yl)amino) propanoic acid,3-((1,3-dioxo-1,3-dihydroisobenzofuran-5-yl)(ethyl)amino)propanoic acid,3-((3,5-bis(chlorocarbonyl) phenyl)amino) propanoic acid,3-((3,5-bis(chlorocarbonyl) phenyl)(ethyl)amino) propanoic acid,4-(4-(chlorocarbonyl)phenyl)-4-oxobutanoic acid,4-(3,5-bis(chlorocarbonyl)phenyl)-4-oxobutanoic acid,4-(1,3-dioxo-1,3-dihydroisobenzofuran-5-yl)-4-oxobutanoic acid,2-(3,5-bis(chlorocarbonyl) phenyl)acetic acid,2-(2,4-bis(chlorocarbonyl)phenoxy) acetic acid,4-((3,5-bis(chlorocarbonyl) phenyl)amino)-4-oxobutanoic acid,2-((3,5-bis(chloro carbonyl)phenyl)amino)acetic acid,2-(N-(3,5-bis(chlorocarbonyl)phenyl)acetamido)acetic acid,2,2′-((3,5-bis(chlorocarbonyl)phenylazanediyl) diacetic acid,N-[(1,3-dihydro-1,3-dioxo-5-isobenzofuranyl)carbonyl]-glycine,4-[[(1,3-dihydro-1,3-dioxo-5-isobenzofuranyl)carbonyl]amino]-benzoicacid, 1,3-dihydro-1,3-dioxo-4-isobenzofuran propanoic acid,5-[[(1,3-dihydro-1,3-dioxo-5-isobenzofuranyl)carbonyl]amino]-1,3-benzenedicarboxylicacid and 3-[(1,3-dihydro-1,3-dioxo-5-isobenzofuranyl)sulfonyl]-benzoicacid.

Another embodiment is represented by Formula (III).

wherein the carboxylic acid group may be located meta, para or orthoupon the phenyl ring.

Representative examples where the hydrocarbon moiety is an aliphaticgroup are represented by Formula (IV):

wherein X is a halogen (preferably chlorine) and n is an integer from 1to 20, preferably 2 to 10. Representative species include:4-(chlorocarbonyl) butanoic acid, 5-(chlorocarbonyl) pentanoic acid,6-(chlorocarbonyl) hexanoic acid, 7-(chlorocarbonyl) heptanoic acid,8-(chlorocarbonyl) octanoic acid, 9-(chlorocarbonyl) nonanoic acid,10-(chlorocarbonyl) decanoic acid, 11-chloro-11-oxoundecanoic acid,12-chloro-12-oxododecanoic acid, 3-(chlorocarbonyl)cyclobutanecarboxylicacid, 3-(chlorocarbonyl)cyclopentane carboxylic acid,2,4-bis(chlorocarbonyl)cyclopentane carboxylic acid,3,5-bis(chlorocarbonyl) cyclohexanecarboxylic acid, and4-(chlorocarbonyl) cyclohexanecarboxylic acid. While the acyl halide andcarboxylic acid groups are shown in terminal positions, one or both maybe located at alternative positions along the aliphatic chain. While notshown in Formula (IV), the acid-containing monomer may includeadditional carboxylic acid and acyl halide groups.

Representative examples of acid-containing monomers include at least oneanhydride group and at least one carboxylic acid groups include:3,5-bis(((butoxycarbonyl)oxy)carbonyl)benzoic acid,1,3-dioxo-1,3-dihydroisobenzofuran-5-carboxylic acid,3-(((butoxycarbonyl)oxy)carbonyl)benzoic acid, and4-(((butoxycarbonyl)oxy)carbonyl)benzoic acid.

The upper concentration range of acid-containing monomer may be limitedby its solubility within the non-polar solution. In most embodiments,the upper concentration limit is less than 1 wt %. In one set ofembodiments, the acid-containing monomer is provided in the non-polarsolution at concentration of at least 0.01 wt %, 0.02 wt %, 0.03 wt %,0.04 wt %, 0.05 wt %, 0.06 wt %, 0.07 wt %, 0.08 wt %, 0.1 wt % or even0.13 wt % while remaining soluble in solution. In another set ofembodiments, the non-polar solution comprises from 0.01 to 1 wt %, 0.02to 1 wt %, 0.04 to 1 wt % or 0.05 to 1 wt % of the acid-containingmonomer. The inclusion of the acid-containing monomer during interfacialpolymerization between the polyfunctional amine and acyl halide monomersresults in a membrane having improved performance. And, unlike posthydrolysis reactions that may occur on the surface of the thin-filmpolyamide layer, the inclusion of the acid-containing monomer duringinterfacial polymerization is believed to result in a polymer structurethat is beneficially modified throughout the thin-film layer.

In a preferred embodiment, the thin film polyamide layer ischaracterized by having a dissociated carboxylate content of at least0.18, 0.20, 0.22, 0.3, 0.40 and in some embodiments at least 0.45moles/kg of polyamide at pH 9.5 as measured by a RutherfordBackscattering (RBS) measurement technique. More specifically, samplesmembranes (1 inch×6 inch) are boiled for 30 minutes in deionized water(800 mL), then placed in a 50/50 w/w solution of methanol and water (800mL) to soak overnight. Next, 1 inch×1 inch size sample of thesemembranes are immersed in a 20 mL 1×10⁻⁴ M AgNO₃ solution with pHadjusted to 9.5 for 30 minutes. Vessels containing silver ions arewrapped in tape and to limit light exposure. After soaking with thesilver ion solution, the unbound silver is removed by soaking themembranes in 2 clean 20 mL aliquots of dry methanol for 5 minutes each.Finally, the membranes are allowed to dry in a nitrogen atmosphere for aminimum of 30 minutes. Membrane samples are mounted on a thermally andelectrically conductive double sided tape, which was in turn mounted toa silicon wafer acting as a heat sink The tape is preferably ChromericsThermattach T410 or a 3M copper tape. RBS measurements are obtained witha Van de Graff accelerator (High Voltage Engineering Corp., Burlington,Mass.); A 2 MeV He⁺ room temperature beam with a diameter of 3 mm at anincident angle of 22.5°, exit angle of 52.5°, scattering angle of 150°,and 40 nanoamps (nAmps) beam current. Membrane samples are mounted ontoa movable sample stage which is continually moved during measurements.This movement allows ion fluence to remain under 3×10¹⁴ He⁺/cm².Analysis of the spectra obtained from RBS is carried out using SIMNRA®,a commercially available simulation program. A description of its use toderive the elemental composition from RBS analysis of RO/NF membranes isdescribed by; Coronell, et. al. J. of Membrane Sci. 2006, 282, 71-81 andEnvironmental Science & Technology 2008, 42(14), 5260-5266. Data can beobtained using the SIMNRA® simulation program to fit a two layer system,a thick polysulfone layer beneath a thin polyamide layer, and fitting athree-layer system (polysulfone, polyamide, and surface coating) can usethe same approach. The atom fraction composition of the two layers(polysulfone before adding the polyamide layer, and the surface of finalTFC polyamide layer) is measured first by XPS to provide bounds to thefit values. As XPS cannot measure hydrogen, an H/C ratio from theproposed molecular formulas of the polymers were used, 0.667 forpolysulfone and a range of 0.60-0.67 was used for polyamide. Althoughthe polyamides titrated with silver nitrate only introduces a smallamount of silver, the scattering cross section for silver issubstantially higher than the other low atomic number elements (C, H, N,O, S) and the size of the peak is disproportionately large to the othersdespite being present at much lower concentration thus providing goodsensitivity. The concentration of silver is determined using the twolayer modeling approach in SIMNRA® by fixing the composition of thepolysulfone and fitting the silver peak while maintaining a narrowwindow of composition for the polyamide layer (layer 2, rangespredetermined using XPS). From the simulation, a molar concentration forthe elements in the polyamide layer (carbon, hydrogen, nitrogen, oxygenand silver) is determined. The silver concentration is a directreflection of the carboxylate molar concentration available for bindingsilver at the pH of the testing conditions. The moles of carboxylicacids groups per unit area of membrane is indicative of the number ofinteractions seen by a species passing through the membrane, and alarger number will thus favorably impact salt passage. This value may becalculated by multiplying the measured carboxylate content by a measuredthickness and by the polyamide density. Alternatively, the carboxylatenumber per unit area of membrane (moles/m2) may be determined moredirectly by methods that measure the total complexed metal within aknown area. Approaches using both Uranyl acetate and toluidine blue Odye are described in: Tiraferri, et. al., Journal of Membrane Science,2012, 389, 499-508. An approach to determine the complexed cation(sodium or potassium) content in membranes by polymer ashing isdescribed in (Wei Xie, et al., Polymer, Volume 53, Issue 7, 22 Mar.2012, Pages 1581-1592).

A preferred method to determine the dissociated carboxylate number at pH9.5 per unit area of membrane for a thin film polyamide membrane is asfollows. A membrane sample is boiled for 30 minutes in deionized water,then placed in a 50 wt % solution of methanol in water to soakovernight. Next, the membrane sample is immersed in a 1×10⁻⁴M AgNO₃solution with pH adjusted to 9.5 with NaOH for 30 minutes. After soakingin the silver ion solution, the unbound silver is removed by soaking themembranes twice in dry methanol for 30 minutes. The amount of silver perunit area is preferably determined by ashing, as described by Wei, andredissolving for measurement by ICP. Preferably, the dissociatedcarboxylate number at pH 9.5 per square meter of membrane is greaterthan 6×10⁻⁵, 8×10⁻⁵, 1×10⁻⁴, 1.2×10⁻⁴, 1.5×10⁻⁴, 2×10⁻⁴, or even 3×10⁻⁴moles/m².

In yet another preferred embodiment, the thin film layer has anisoelectric point (IEP) of less than or equal to 4.3, 4.2, 4.1, 4, 3.8,3.6 or in some embodiments 3.5. The isoelectric point can be determinedusing a standard Zeta-Potential technique with a quartz cell byelectrophoretic light scattering (ELS) using Desal Nano HS instrument.For example, membrane samples (2 inch×1 inch) are first boiled for 20minutes in DI water, then rinsed well with room temperature DI water andstored at room temperature in a fresh DI solution overnight. The samplesare then loaded as per reference: 2008 “User's Manual for the Delsa™Nano Submicron Particle Size and Zeta Potential,” and the “Pre-CourseReading” for the same instrument presented by Beckmann Coulter. pHtitration is completed over a range from pH 10 to pH 2 and isoelectricpoint is determined at the pH where the zeta potential becomes zero.

The non-polar solution may include additional constituents includingco-solvents, phase transfer agents, solubilizing agents, complexingagents and acid scavengers wherein individual additives may servemultiple functions. Representative co-solvents include: benzene,toluene, xylene, mesitylene, ethyl benzene. diethylene glycol dimethylether, cyclohexanone, ethyl acetate, butyl carbitol™ acetate, methyllaurate and acetone. A representative acid scavenger includes N,N-diisopropylethylamine (DIEA). The non-polar solution may also includesmall quantities of water or other polar additives but preferably at aconcentration below their solubility limit in the non-polar solution.

Once brought into contact with one another, the polyfunctional acylhalide and polyfunctional amine monomers react at their surfaceinterface to form a polyamide layer or film. This layer, often referredto as a polyamide “discriminating layer” or “thin film layer,” providesthe composite membrane with its principal means for separating solute(e.g. salts) from solvent (e.g. aqueous feed). The reaction time of thepolyfunctional acyl halide and the polyfunctional amine monomer may beless than one second but contact times typically range from about 1 to60 seconds. The removal of the excess solvent can be achieved by rinsingthe membrane with water and then drying at elevated temperatures, e.g.from about 40° C. to about 120° C., although air drying at ambienttemperatures may be used. However, for purposes of the presentinvention, the membrane is preferably not permitted to dry and is simplyrinsed (e.g. dipped) with water and optionally stored in a wet state.Once formed, the polyamide layer is treated by applying an aqueoussolution of nitrous acid. A variety of techniques are described in U.S.Pat. No. 4,888,116 and are incorporated herein by reference. It isbelieved that the nitrous acid reacts with the residual primary aminegroups present in the polyamide discrimination layer to form diazoniumsalt groups. At least a portion of these diazonium salt groups hydrolyzeto form phenol groups or azo crosslinks via diazo-coupling. Although theaqueous solution may include nitrous acid, it preferably includesreagents that form nitrous acid in situ, e.g. an alkali metal nitrite inan acid solution or nitrosyl sulfuric acid. Because nitrous acid isvolatile and subject to decomposition, it is preferably formed byreaction of an alkali metal nitrite in an acidic solution in contactwith the polyamide discriminating layer. Generally, if the pH of theaqueous solution is less than about 7, (preferably less than about 5),an alkali metal nitrite will react to liberate nitrous acid. Sodiumnitrite reacted with hydrochloric or sulfuric acid in an aqueoussolution is especially preferred for formation of nitrous acid. Theaqueous solution may further include wetting agents or surfactants. Theconcentration of the nitrous acid in the aqueous solution is preferablyfrom 0.01 to 1 wt %. Generally, the nitrous acid is more soluble at 5°than at 20° C. and somewhat higher concentrations of nitrous acid areoperable at lower temperatures. Higher concentrations are operable solong as the membrane is not deleteriously affected and the solutions canbe handled safely. In general, concentrations of nitrous acid higherthan about one-half (0.5) percent are not preferred because ofdifficulties in handling these solutions. Preferably, the nitrous acidis present at a concentration of about 0.1 weight percent or lessbecause of its limited solubility at atmospheric pressure. Thetemperature at which the membrane is contacted can vary over a widerange. Inasmuch as the nitrous acid is not particularly stable, it isgenerally desirable to use contact temperatures in the range from about0° to about 30° C., with temperatures in the range from 0° to about 20°C. being preferred. Temperatures higher than this range can increase theneed for ventilation or super-atmospheric pressure above the treatingsolution. Temperatures below the preferred range generally result inreduced reaction and diffusion rates.

The reaction between the nitrous acid and the primary amine groupsoccurs relatively quickly once the nitrous acid has diffused into themembrane. The time required for diffusion and the desired reaction tooccur will depend upon the concentration of nitrous acid, anypre-wetting of the membrane, the concentration of primary amine groupspresent and the temperature at which contact occurs. Contact times mayvary from a few minutes to a few days. The optimum reaction time can bereadily determined empirically for a particular membrane and treatment.

One preferred application technique involves passing the aqueous nitrousacid solution over the surface of the membrane in a continuous stream.This allows the use of relatively low concentrations of nitrous acid.When the nitrous acid is depleted from the treating medium, it can bereplenished and the medium recycled to the membrane surface foradditional treatment. Batch treatments are also operable. The specifictechnique for applying aqueous nitrous acid is not particularly limitedand includes spraying, film coating, rolling, or through the use of adip tank among other application techniques. Once treated the membranemay be washed with water and stored either wet or dry prior to use. ForRO and NF applications, membranes treated with nitrous acid preferablyhave a NaCl rejection of at least 2% when tested using an aqueous NaClsolution (250 ppm) at 25° C. and 70 psi.

A representative reaction scheme illustrating the treatment of thepolyamide with nitrous acid is provided below.

Representative Diazonium Reaction Scheme:

The thin film polyamide layer may optionally include hygroscopicpolymers upon at least a portion of its surface. Such polymers includepolymeric surfactants, polyacrylic acid, polyvinyl alcohol, polyvinylacetate, polyalkylene oxide compounds, poly(oxazoline) compounds,polyacrylamides and related reaction products as generally described inU.S. Pat. No. 6,280,853; U.S. Pat. No. 7,815,987; U.S. Pat. No.7,918,349 and U.S. Pat. No. 7,905,361. In some embodiments, suchpolymers may be blended and/or reacted and may be coated or otherwiseapplied to the polyamide membrane from a common solution, or appliedsequentially.

Many embodiments of the invention have been described and in someinstances certain embodiments, selections, ranges, constituents, orother features have been characterized as being “preferred.”Characterizations of “preferred” features should in no way beinterpreted as deeming such features as being required, essential orcritical to the invention.

EXAMPLES

Sample membranes were prepared using a pilot scale membranemanufacturing line. Polysulfone supports were casts from 16.5 wt %solutions in dimethylformamide (DMF) and subsequently soaked in anaqueous solution meta-phenylene diamine (mPD). The resulting support wasthen pulled through a reaction table at constant speed while a thin,uniform layer of a non-polar coating solution was applied. The non-polarcoating solution included a isoparaffinic solvent (ISOPAR L), acombination of trimesoyl acid chloride (TMC), and/or1-carboxy-3,5-dichloroformyl benzene (mhTMC) in varying ratios with andwithout mesitylene. Excess non-polar solution was removed and theresulting composite membrane was passed through water rinse tanks anddrying ovens. Sample membrane sheets were then either (i) stored indeionized water until testing; or (ii) soaked for approximately 15minutes in a solution at 0-10 ° C. prepared by combining 0.05% w/v NaNO₂and 0.1 w/v % HCl and thereafter rinsed and stored in deionized wateruntil testing.

Example 1

Testing was conducted with a 2000 ppm NaCl solution at room temperatureand 150 psi. Samples subjected to post-treatment with nitrous acid aredesigned with an asterisk (*). The results are summarized in Table 1.“SP” refers to NaCl passage. As shown by the test data, post-treatmentof samples including mesitylene demonstrated an unexpected improvementin flux.

TABLE 1 Mean mPD TMC Mesitylene (AvgFlux) Mean % change in % changeSample (wt %) (wt %) (vol %) GFD (Avg SP) Flux in SP  1-1a 2.5 0.25 012.8 0.95% *1-1b 2.5 0.25 0 13.5 2.11% 5.3 121.1  1-1c 2.5 0.25 10 31.10.51% *1-1d 2.5 0.25 10 41.5 0.97% 33.3 90.7  1-2a 3.5 0.25 0 17.0 0.75%*1-2b 3.5 0.25 0 20.7 1.31% 21.6 75.6  1-2c 3.5 0.25 10 30.9 0.43% *1-2d3.5 0.25 10 47.2 0.75% 52.7 75.1  1-3a 4.5 0.25 0 20.9 0.57% *1-3b 4.50.25 0 27.3 1.21% 30.3 113.5  1-3c 4.5 0.25 10 23.9 0.45% *1-3d 4.5 0.2510 39.6 0.69% 65.6 53.9

Example 2

Testing was conducted with a 2000 ppm NaCl solution at room temperatureand 150 psi. The results are summarized in Table 2. As shown by the testdata, post-treatment of samples including mesitylene and anacid-containing monomer (mhTMC) had an unexpected improvement in saltpassage as compared with membranes without post-treatment, or those withpost treatment but without mesitylene and an acid-containing monomer.

TABLE 2 Mean mPD TMC Mesitylene mhTMC (AvgFlux) Mean % change in %change Sample (wt %) (wt %) (v0l %) (wt %) GFD (Avg SP) Flux in SP  2-1a2.5 0.25 0 0 12.8 0.95% *2-1b 2.5 0.25 0 0 13.5 2.11% 5.3 121.1  2-1c2.5 0.22 10 0.03 33.9 0.46% *2-1d 2.5 0.22 10 0.03 37.8 0.48% 11.5 4.3 2-2a 3.5 0.25 0 0 17.0 0.75% *2-2b 3.5 0.25 0 0 20.7 1.31% 21.6 75.6 2-2c 3.5 0.22 10 0.03 31.5 0.36% *2-2d 3.5 0.22 10 0.03 41.3 0.37% 31.13.7  2-3a 4.5 0.25 0 0 20.9 0.57% *2-3b 4.5 0.25 0 0 27.3 1.21% 30.3113.5  2-3c 4.5 0.22 10 0.03 24.5 0.66% *2-3d 4.5 0.22 10 0.03 32.00.38% 30.8 −42.9

Example 3

Testing was conducted with a 2000 ppm NaCl solution at room temperatureand 150 psi. The results are summarized in Table 3. As shown by the testdata, post-treatment of samples including mesitylene and anacid-containing monomer (mhTMC) had an unexpected improvement in saltpassage (SP) for membranes prepared with increasing quantities of theacid-containing monomer (mhTMC).

TABLE 3 Mean (Avg mPD TMC Mesitylene mhTMC Flux) Mean Sample (wt %) (wt%) (vol %) (wt %) GFD (Avg SP)  3a 3.5 0.25 0 0.00 30.4 0.47% *3b 3.50.25 0 0.00 40.6 0.61%  3c 3.5 0.24 10 0.01 32.2 0.41% *3d 3.5 0.24 100.01 44.9 0.50%  3e 3.5 0.20 0 0.05 33.3 0.39% *3f 3.5 0.20 0 0.05 44.70.40%  3g 3.5 0.17 10 0.08 34.1 0.71% *3h 3.5 0.17 10 0.08 46.8 0.46%

1. A method for making a composite polyamide membrane comprising aporous support and a thin film polyamide layer, wherein the methodcomprises: i) applying a polar solution comprising a polyfunctionalamine monomer and a non-polar solution comprising a polyfunctional acylhalide monomer to a surface of a porous support and interfaciallypolymerizing the monomers to form a thin film polyamide layer, whereinthe non-polar solution comprises at least 50 vol % of a C₅ to C₂₀aliphatic hydrocarbon and from 2 to 25 vol % of benzene or benzenesubstituted with one or more C₁ to C₆ alkyl groups; and ii) applying anaqueous solution of nitrous acid to the thin film polyamide layer. 2.The method of claim 2 wherein the non-polar solution comprises1,3,5-trimethylbenzene.
 3. The method of claim 2 wherein the non-polarsolution comprises from 50 to 90 wt % of a paraffin or isoparaffin, orcombination thereof.
 4. The method of claim 1 wherein the non-polarsolution further comprises an acid-containing monomer comprising aC₂-C₂₀ hydrocarbon moiety substituted with at least one carboxylic acidfunctional group or salt thereof and at least one amine-reactivefunctional group selected from: acyl halide, sulfonyl halide andanhydride, wherein the acid-containing monomer is distinct from thepolyfunctional acyl halide monomer.
 5. The method of claim 4 wherein theacid-containing monomer comprises an arene moiety.
 6. The method ofclaim 4 wherein the acid-containing monomer comprises an aliphaticmoiety.
 7. The method of claim 4 wherein the acid-containing monomercomprises at least two amine-reactive functional groups.
 8. The methodof claim 4 wherein the thin film polyamide layer has a dissociatedcarboxylic acid content of at least 0.18 moles/kg at pH 9.5 as measuredby RBS prior to the step of applying the aqueous solution of nitrousacid.
 9. The method of claim 4 wherein the thin film polyamide layer hasa dissociated carboxylic acid content of at least 0.45 moles/kg at pH9.5 prior to the step of applying the aqueous solution of nitrous acid.10. The method of claim 4 wherein the thin film polyamide layer has anisoelectric point (IEP) of less than or equal to 4.3 prior to the stepof applying the aqueous solution of nitrous acid.